During the past 30 years we have developed a comprehensive multidisciplinary research program using the equine infectious anemia virus (EIAV) system to examine the fundamental mechanisms by which lentiviruses persist despite robust host immune responses and to evaluate experimental immunization strategies as models for HIV-1 infection and vaccine development. In the previous grant period, we demonstrated for the first time that Env variation is indeed a primary determinant of lentivirus vaccine efficacy that will need to be addressed in the effort to develop broadly protective vaccines. In the current competitive renewal application we propose to extend these studies to test our central hypothesis that EIAV Env is the primary determinant of vaccine efficacy and that effective vaccines must elicit appropriate broadly reactive immunity against diverse virus strains. Moreover, we suggest that Env antigen and its method of presentation need to be optimized to elicit enduring broadly protective immunity. Thus, the following complementary specific aims are proposed: (i) to define the Env determinants of vaccine protection and to characterize the specificity of vaccine immunity to these critical determinants, (ii) to characterize the maturation of immune responses to attenuated EIAV vaccines that is associated with the development of enduring protective vaccine immunity, and (iii) to develop and evaluate novel immunization procedures using multivalent and consensus Env immunogens for their ability to elicit broadly protective immunity to diverse EIAV strains. In the first specific aim, we will use selected chimeric Env viruses derived from two defined variant Env species that differ markedly in vaccine protection to map specific Env determinants of protection based on experimental challenge of ponies immunized with a reference attenuated EIAV vaccine. In the second specific aim, we will perform a complementary study to define the immune correlates of vaccine efficacy by characterizing the Env-specific antibody and cellular immune reponses that distinguish nonprotective and protective vaccine immunity. In the third specific aim, we will evaluate a series of vaccine modalities (attenuated virus, virus like particles, and adenovirus vectors) expressing either a mixture of variant Env species or a consensus Env for their ability to produce broadly reactive vaccine immunity and to protect against diverse Env strains of EIAV in experimentally immunized ponies. It is anticiapated that the results of these studies will provide novel insights into the fundamental mechanisms by which Env variation can circumvent protective vaccine immunity and determine the potential of alternative vaccine strategies to overcome the challenge of Env diversity in vaccine development. Thus, these EIAV studies address critical issues in AIDS vaccine research and can provide important information relevant to the design of candidate human AIDS vaccines.